This invention relates in general to vehicle transmissions and in particular to a method and apparatus for automatically controlling the operation of an auxiliary section of a compound transmission.
In virtually all land vehicles in use today, a transmission is provided in a drive train between a source of rotational power, such as an internal combustion or diesel engine, and the driven axle and wheels of the vehicle. A typical transmission includes a case containing an input shaft, an output shaft, and a plurality of meshing gears. Means are provided for connecting selected ones of the meshing gears between the input shaft and the output shaft to provide a desired speed reduction gear ratio therebetween. The meshing gears contained within the transmission case are of varying size so as to provide a plurality of such gear ratios. By appropriately shifting among these various gear ratios, acceleration and deceleration of the vehicle can be accomplished in a smooth and efficient manner.
A compound transmission is a transmission that is divided into two or more separate sections, each of which provides a plurality of gear ratios. Typically, a compound transmission is divided into two sections, namely, a main section that provides a first plurality of gear ratios and an auxiliary section that provides a second plurality of gear ratios. The total number of gear ratios available from the compound transmission as a whole is equal to the product of the number of gear ratios available from the main section and the number of gear ratios available from the auxiliary section. Thus, if the main section provides four gear ratios and the auxiliary section provides two gear ratios, the compound transmission as a whole can provide a total of eight gear ratios.
The auxiliary sections of compound transmissions can be divided into two general types, namely, range type and splitter type. In a range type of compound transmission, the differences between the adjacent gear ratios provided by the auxiliary section are greater than the differences between the adjacent gear ratios provided by the main section. Thus, to shift sequentially upwardly through the available gear ratios, the main section is initially shifted through all of its gear ratios while the range section is maintained in a first gear ratio, then the range section is shifted into a second gear ratio before re-shifting through all of the gear ratios of the main section. In a splitter type of compound transmission, the differences between the adjacent gear ratios provided by the auxiliary section are less than the differences between the adjacent gear ratios provided by the main section. Thus, to shift sequentially upwardly through the available gear ratios, the main section is maintained in a first gear ratios while the splitter section is initially shifted through all of its gear ratios, then the main section is shifted into a second gear ratio before re-shifting through all of the gear ratios of the splitter section.
Many transmission structures are known in the art for performing these gear ratio selections manually, i.e., in response to some physical exertion by the driver of the vehicle. In a conventional manual transmission, the driver grasps and moves an upper portion of a pivotable shift lever to effect shifting of the main section of the compound transmission. In response thereto, a lower portion of the shift lever is moved within the main section of the compound transmission to select the desired gear ratio therein. To control the operation of the auxiliary section, it is known to provide a thumb switch on the upper end of the shift lever. The thumb switch is manually moved by the driver when it is desired to select a gear ratio for the auxiliary section. Movement of the thumb switch activates a pneumatic actuator to effect the desired shifting of the auxiliary section. Manually operated transmissions of this general type are well known in the art and are relatively simple, inexpensive, and lightweight in structure and operation. Because of this, the majority of medium and heavy duty truck transmissions in common use today are manually operated.
More recently, however, in order to improve the convenience of use of manually operated transmissions, various structures have been proposed for partially or fully automating the shifting of an otherwise manually operated transmission, in either or both of the main and auxiliary sections. In a partially or fully automated manual transmission, some or all of the physical exertions mentioned above may be eliminated by providing an electronic control system for automatically determining when a shifting operation should occur and appropriate mechanical actuators for effecting such shifting. Although fully automated transmissions offer the advantages of increased convenience of use and reduced driver fatigue, manual transmissions provide the advantages of decreased cost, increased torque capacity, and better fuel economy. Thus, the majority of medium and heavy duty truck transmissions in common use today are manual transmissions.
Unfortunately, in manually shifted compound transmissions, it has been found that the driver of the vehicle can operate the thumb switch for the auxiliary section in such a manner as to cause premature wear and potential failure thereof. Specifically, if the auxiliary section of the compound transmission is attempted to be shifted when the vehicle speed is greater than a predetermined maximum limit, then damage can occur to some of the internal shifting components of the auxiliary section. Thus, it would be desirable to provide an improved method and apparatus for operating a compound transmission wherein a manually selected shift of the auxiliary section is automatically prevented from occurring unless the vehicle speed is within a predetermined maximum limit or range.